Squirrel-cage induction motors are widely used in the agricultural, commercial, municipal, and residential sectors due to their high energy efficiency, reliability, and good controllability. As induction motors become more involved in critical tasks, such as heating, ventilating, and air conditioning (HVAC) systems used in places like hospital intensive-care units (ICU) and energy-efficient buildings, accurate and reliable condition monitoring of their status is assuming a greater importance.
Many induction motor condition monitoring applications require accurate knowledge of instantaneous rotor speed. For example, precise knowledge of the instantaneous rotor speed can be essential for real-time tracking of an induction motor's rotor temperature. The instantaneous rotor speed information may also be required when estimating induction motor electrical parameters, such as stator inductance and total leakage factor. Furthermore, instantaneous rotor speed information can also be used when assessing in-service induction motor's energy efficiency in a non-intrusive manner.
In the absence of dedicated speed sensors, the instantaneous rotor speed can be estimated by extracting a rotor-speed-dependent instantaneous rotor slot harmonic frequency from electrical current measurements. For example, some approaches are based on a superheterodyne principle and use a fixed nominal rotor slot harmonic frequency as a carrier frequency, and a predetermined filter bandwidth when demodulating the instantaneous rotor slot harmonic frequency signal. This approach is generally limited to motor applications involving a fixed average load and hence fixed average rotor speed, or motor applications with moderate rotor speed oscillation superimposed on the fixed average rotor speed.
However, many motor applications, such as conveyor belt applications in postal sorting offices and airport terminals, involve significant rotor speed fluctuations. In these types of applications, the average rotor speed is no longer a constant value. Rotor speed fluctuations may cause rotor-speed-dependent instantaneous rotor slot harmonic frequency to deviate considerably from the fixed nominal rotor slot harmonic frequency. In addition, many other motor applications, such as applications in refrigeration and industrial processes with reciprocating compressors, may contain pulsating speed components whose frequencies are larger than the predetermined filter bandwidth in the rotor slot harmonics. Use of digital filters with predetermined filter bandwidth may result in the exclusion of these pulsating components in the rotor slot harmonics. Both rotor speed fluctuations and load-induced rotor speed pulsations lead to distortions in the estimated instantaneous rotor slot harmonic frequency, and hence distortions in an estimated instantaneous rotor speed.